1. Field of the Invention
This invention relates generally to hard disk drives (HDDs) and more particularly to an HDD with a servo control system that is capable of performing “just-in-time” (JIT) seeks of the read/write transducer to a target data track.
2. Description of the Related Art
Disk drives are digital data storage devices that allow host computer systems to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes one or more magnetic recording disks that are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write transducers is disposed adjacent to surfaces of the disks to transfer data between the disks and a host computer. The transducers are radially positioned over the disks by a rotary actuator and a closed loop servo control system.
Each disk surface has a plurality of concentric data tracks. A preamplifier/write driver circuit generates write currents to the transducer to selectively magnetize the tracks during a data write operation, and amplifies read signals generated by the transducer from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are connected to the preamplifier/write driver circuit to transfer the data between the disks and the host computer.
The servo system can operate in two primary modes: seeking and track following. During a seek, a selected transducer is moved from an initial track to a target track on the corresponding disk surface. The servo system applies current to an actuator to first accelerate and then decelerate the transducer toward the target track. As the transducer approaches the target track, the servo system initiates a settle mode to bring the transducer to rest over the target track within a selected settle threshold, such as a percentage of the track width from track center. Thereafter, the servo system enters the track following mode wherein the transducer is nominally maintained over the center of the target track until another seek is performed.
In early disk drives the actuator operated at full power and moved the transducer with maximum allowable velocity to minimize the seek time for all seek lengths. Once the seek is completed and the transducer reaches the target track, additional time is required to rotate the disk until the target sector is beneath the transducer. This additional time is referred to as rotational latency. More recently, a “just-in-time” (JIT) seek method was implemented. With the JIT method, the actuator takes advantage of the rotational latency so that the transducer arrives at the target track just before the target sector has rotated to below the transducer. This increases the seek time but minimizes power consumption.
To implement the JIT method, the servo system clips or limits the transducer velocity, based on the value of rotational latency. Thus a JIT seek is a seek with a limited velocity lower than the maximum allowable velocity. One problem with this JIT method is that it may not affect all long seeks (seeks longer than some predetermined number of tracks), since some long seeks may already have a maximum velocity less than the limited velocity. Therefore, JIT does not affect those seeks. Power is not saved on seeks which are not affected by a reduction of the maximum velocity.
What is needed is a HDD with a servo control system with an improved JIT method that minimizes power for all long seeks.